1. Field of the Invention
The present invention relates to an audio decoding device for forming an audio signal by decoding coded signals transmitted by multi-channels of, for example, MPEG2 , and also to a signal processing device for performing decoding by synchronizing audio compression signals such as MPEG audio signals or Dolby.RTM. AC-3 signals between an internal decoder and an external decoder.
2. Description of the Related Art
In MPEG2, for example, audio signals to be output simultaneously are transmitted through multi-channels.
In a bit stream by which the multi-channel signals are transmitted, channels for audio signals to be reproduced simultaneously are not proximate to each other and are arranged discretely timewise.
In order to decode and output the coded signals for the multi-channels in such a bit stream, signal decoding should be performed frame by frame as seen in an MPEG2 multi-channel decoder program provided by, for example, the FTP site of the University of California at Berkeley.
Alternatively, the coded signals on the channels arranged in the first half of the frame are temporarily stored in a memory, and then these coded signals are decoded while the coded signals an the channels arranged in the second half of the frame are decoded.
In either case, a large amount of coded signals need to be temporarily stored in a memory, which raises the required capacity of the memory and thus increases the size of the device.
Moreover, in order to perform the operation for decoding the coded signals, a memory capable of high-speed access should be used for temporarily storing the coded signals used for the operation. Since such a memory is expensive, reduction in the required capacity of the memory has been strongly demanded.
Regarding the necessity of a memory capable of high-speed access, a conventional decoding device 500 will be described with reference to FIG. 17.
The decoding device 500 includes a memory section 510 and an operation section 520. The operation section 520 includes a sub-band signal generation section 521 and a sub-band synthesis section 522. When coded signals for n channels are given, the operation section 520 decodes the coded signals into sub-band signals. The sub-band signals for each channel is processed with a sub-band synthesis filter operation to generate an audio signal to be output. The memory section 510 includes a high-speed access memory such as an SRAM, which includes memory areas 511 through 514 for storing sub-band synthesis filter data and a memory area 515 for storing the sub-band signal data.
The decoding device 500 having the above-described structure operates in the following manner.
When the coded signals are input to the operation section 520, the sub-band signal generation device 521 decodes the coded signals into sub-band signals and temporarily stores the sub-band signals in the memory area 515. Then the sub band synyhesis section 522 reads the sub-band signals from the memory area 515 and performs a sub-band synthesis filter operation of the sub-band signals. Thus, an audio signal is generated and output.
The sub-band synthesis filter data in the memory areas 511 through 514 is partially updated by the sub-band synthesis filter data generated from the sub-band signals in the memory area 515. Accordingly, the operation filter section 520 needs to read the sub-band sythesis data from the memory section 510 when performing the sub-band synthesis filter operation and write again the sub-band synthesis filter data to the memory section 510 after the operation.
The memory section 510 needs to be a memory capable of high-speed access. In the case where the decoding device 500 corresponds to multi-channels, e.g., four channels, the memory section 510 needs to have four memory areas 511 through 514 so as to store sub-band synthesis filter data for the four channels.
A high-speed access memory such as an SRAM used for the memory section 510 needs to have a sufficiently large memory capacity to retain the sub-band synthesis filter data for at least four channels in order to perform real-time reproduction of four-channel data. Such a memory, which is generally expensive, significantly increases the cost of the audio decoding devices.
One format for digital audio interface is the IEC958 format. The IEC958 format is common for industrial and consumer uses. The sub-frame format thereof includes an area in which a synchronous preamble, Aux, 20-bit-unit audio data and other data can be added.
In the case of 2-channel transmission, a frame is formed by alternately repeating sub-frames for each of the two channels, and a block is formed of first through 192nd frames and then transmitted. The format of the audio data to be inserted into the sub-frames is not standardized. For example, a PCM signal formed by sampling an analog audio signal or a non-PCM signal (defined by the format described in ISO/IEC EC 11172-3:1993 and 13818-3:1996) such as compression audio data formed by sub-band coding.
In the case of reproducing coded signals for multi-channels (e.g., five channels of right forward, left forward, center, right rearward and left rearward) by an audio reproduction apparatus including a decoder, the coded signals for all the channels cannot always be decoded only by a built-in decoder (referred to as the "internal decoder"). Many general audio reproduction apparatuses output only audio signals in the right forward and left forward channels from the internal decoder. Such audio reproduction apparatuses require a separate decoder or a separate audio reproduction apparatus (referred to as the "external decoder") in order to decode the coded signals for the other channels.
In order to synchronize an audio signal output from the internal decoder and an audio signal output from the external decoder while transmitting a non-PCM signal of the IEC958 format from the internal decoder to the external decoder, a conventional audio reproduction apparatus requires a buffer memory for performing synchronization between the internal decoder and the external decoder. Such a buffer memory needs to have a sufficiently large capacity to retain the coded signals corresponding to a transmission delay time.
As can be appreciated from the above description, synchronization between the internal decoder and the external decoder requires a buffer memory therebetween, which inconveniently increases the size of the apparatus.